1. Field of the Invention
The present invention relates to a marine propulsion machine including a vertical drive shaft rotatively driven by an engine, an output gear mechanism driven by the drive shaft, a propeller shaft rotatively driven by the output gear mechanism, and a normally submerging gear case containing the output gear mechanism and the propeller shaft.
2. Description of the Related Art
Marine propulsion machines are disclosed in, for example, JP-A 5-52107 and JP-A 63-97489. The known marine propulsion machine has a gear case holding therein an output gear mechanism driven by a drive shaft rotatively driven by an engine, and a propeller shaft rotatively driven by the output gear mechanism. In this marine propulsion machine, the drive shaft has a first drive shaft interlocked with the engine, and a second drive shaft driven by the first drive shaft to transmit power to the output gear mechanism, and the second drive shaft is disposed on the rear side of the first drive shaft.
The normally submerging gear case has a gearing holding portion holding the output gear mechanism and the propeller shaft, and a support portion extending upward from the gearing holding portion and connected to a case overlying the gear case and having a cross-sectional shape resembling a cross section of a wing. The gearing holding portion has diameter gradually increasing from its front end toward its rear end. If the area of the cross section of the gearing holding portion in a plane perpendicular to a longitudinal direction parallel to a direction in which water flows relative to the ship when the ship moves forward, increases sharply toward the rear, the form drug (hereinafter referred to as “underwater resistance”) resulting from the shape of the gear case while the ship is cruising forward is high, a low-pressure region develops due to the disturbance of water currents when the ship cruises at high cruising speed, and cavitation is liable to occur around a propeller disposed rearward of the gear case.
When an interlocking mechanism included in an operating mechanism for reversing the rotating direction of the propeller shaft is disposed on the front side of the drive shaft in the gear case, and an operating member and an actuating member included in the interlocking mechanism are a pin and a cam eccentric to the center axis of the operating member, the eccentricity of the eccentric pin and the height of the lobe of the cam are determined so as to correspond to a necessary moving distance along the center axis of the propeller shaft. Therefore, the interlocking mechanism has a large dimension with respect to transverse directions, namely, directions perpendicular to the center axis of the propeller shaft in a plane. Consequently, the transverse dimension of a part of the gear case holding the interlocking mechanism is large and the form drag (hereinafter referred to as “underwater resistance”) increases.